Method of treating mounted glass disc to resist breakage



F. s. CRAIG 2,730,840

METHOD OF TREATING MOUNTED GLASS DISC TO RESIST BREAKAGE Jan. 17, 1956Filed Sept. 28, 1949 F|Ca.4. Fig.5. F|c=.7.

INVENTOR.

Ii'EETl-EQD UF-TREA'HIIG MGUNIEB GlLrKSS DISC Eli} RESET ERIKA-RAGEFrank 5. Craig, Chili, N. Y, assignor to General Raiiway Signal Company,Rochester, N. Y.

ApplicationSeptezhioerlfi 149;SriaiNo 118,308

3 (Ilaims. "(CL 49- -79) This invention relates to ring mounted colorglass disc for railway signals, and it'moreparticularly pertains todiscs that have been especially heat treated to resist breakageand-themethod of heat treatment involved.

The conventional Searchlight type ofrailway-signals can display aSelected color aspectby'reason of the-selective positioning of smallcolor glass-discs close to the focal point in the light'beam of "thesignal. "The color discs are secured within metal rings-which are spun,pressed, or otherwise formed over the edges of the glass discs, andthese rings in turn are secured to" an electromagnetically actuatedsector so that they maybe selectively positioned within a beam of lightobtained by a: suitable light system of the signal. Because. ofthe rapidheating and cooling of the color glass 'di'scs astheylare"selectivelypositioned within the light beam, and particularly where approachlighting of the signals ise'rnjgloyed'forv selectively energizing anddeenergizingthe. signals in accordance with the approach of trains, onlya beam of limited intensity can be employed without dangerof breakage ofthe color' glass discs due toh'eatand'thus permittingium colored lightto beemitted by the signal. This'is' more particularly true of the.green color. glass'discs.

An object of the present 'invention is to permit'light beams of higherintensity to beernployed in searchli'ght signals withoutbreakageofthegreen' color'glassdiscs' by reason of high temperature annealing ofthe discs'subsequent to the rings having been secured thereto.

Another object of the; present invention is'toform a diametric crack inthe color glass discs" prior to the mounting of the'discs in rings, andprior tothe high'temperature annealing of the mounted'discs, to furtherprevent breakage of the l'glassin use within-the lightbeam of asearchlight signal.

Other objects,"purposes' and characteristic fe'atures'of the presentinvention will be in partob vious from. the

accompanyingdrawing, and in part pointed out asthe de scriptionprogresses.

In describing the invention in'detail, reference ismade to theaccompanying drawing'in which:

Fig. l is a view of a ring mounteddiametrically cracked color glass'disc formed of'h'eat resistance annealed colored glass; and

Fig. 2 is a plan view of a ringmounted color glass disc of the character'shown in Fig. 1 with shaded portions pro vided to indicate darkportions apparent when the disc is viewed in a polariscope after-high"temperature annealing but prior to being used in a signal;

, Fig. 3 is a plan view of a diametrically cracked-color glass disc asviewed in a; polariscope after the disc has been heat treated'subsequentto mounting within its ring,

and after the disc has been used in alight signal;

Fig. 4 is a' sectional view of a color glass'disc taken along the line4-4 of Fig. 1 after insertion of the disc in'its mounting ring, butprior'tothe ring being spun over the edge of the disc;

FigfS is a, sectional viewof a color disc' after the assoiii EatentedJan. 17, 1956 ice elated mounting ring has beenspunover the edge of thedisc;

Fig. 6 is afragmentary view illustrating "the'manner in which a colorlight disc is disposed inthe" beam of a *searchlight signal;

'dated April 22, 1941, comprises'a light system having a lamp 10 securedin a lamp socketlland'disposed with its filament 12 at the focal pointof a suitable parabolic refiector'13. The reflector 13 focuses the lightemitted by the lamp It) at a point P, and slightly displaced from thisfocal point P is a color' disc 14 secured in a disc operating sector 15by suitable spring clips 16 hearing against a ring 17 in which the colorglass disc 14 is secured. it is generally provided that three of suchcolor glass discs of the colors red, yellow, and green respectively aredisposed adjoining respective openings in this operating sector 15 sothat the electromagnetic actuation of the sector 15 about a pivot point.(not shown) is effective to select the aspect to be'displayed by thesignal in accordance whichever one of the three color glass discs isdisposed within'the beam of light emitted by the light system near thefocal point F.

The green color glass employedis of the commercially available heatresistant lime glass character having a low coefficient of expansion toresist breakage. Glass of this heat resistant character'is annealed soas to remove all internal stresses and thus whenexamined in apolariscope, no stresses are indicated. After a color glass disc-hasbeen cut to a desired diameter fromthis material, a diametric crack 18is formed therein by any suitable means such as the color glass discbeing brought to bear against a hot wire; and then the disc is droppedwithin a flanged ring 17 of soft material such as aluminum or copper asshown in Fig. 4, the open edge 17a of the ring 17 being spun over theedge of the colorglass-disc soas-to-form a'channel shaped ring 17mounting the colorglass disc 14 as is illustrated in the sectionalviewaccording to Fig. 5.

After the color. glass. disc 14 is thus secured within the ring-1'7,subsequent to having its diametric crack 13 formed therein, the mounteddiso14 is inserted in a furnace which has been preheated to 1200 F.,-disposed on a suitable rackwhich supports the disc by its ring 17 anddoes not contact the color. glass.

that internal stresses are set up within the glass, and such stresseshave-been somewhat oriented in a manner to form patterns whenviewed'under the polariscope substantially parallel to the diametriccrack 18.

The ring mounted color glass disc 14 when thus annealed is conditionedtor use-within alight-signal assembled" as schematically illustrated in'Fig. 6, 'andbecause of this particular treatment, operation of thecolor glass disc will withstand a much higher temperature, and permitthe use of a higher wattage lamp in the signal so as to intensify thelight beam emitted, without causing the color disc 14 to be broken byheating within the signal so as to permit pieces of the glass to becomedisassociated from the disc 14 and thus permit the passage of uncoloredlight through the disc 14 and also perhaps damage or obstruct theoperation of the operating mechanism of the signal by the pieces ofglass dropping into such operating mechanism.

After a diametrically cracked and high temperature annealed color glassdisc of the character described has been used in a signal, when removedfrom the signal and viewed in a polariscope, a pattern is apparent as isillustrated in Fig. 3 wherein it is indicated that additional stresseshave been set up because of the heat applied to the glass by the lightbeam of the signal. It will be noted, however, that there are still darkareas indicated when viewed in the polariscope substantially parallel tothe diametric crack along the lines of the shaded areas illustrated inFig. 2 as being indicative of internal stresses that are set up by thehigh temperature heat treatment subsequent to assembly of the ring andprior to its use in a signal.

Although there may be different theories with respect to the reasons forthe improved resistance of the disc 14 to breakage when treated as abovedescribed, it is reasonable to believe that the increased resistance ofthe glass disc 14 to breakage is accomplished by obtaining additionalroom for expansion of the disc 14- within the mounting ring 17 in twodifferent ways. One way is the forming of the diametric crack 18 whichactually expands the disc 14 and thus enlarges the diameter of the ring17 across the points where it is spun over the edge of the color glassdisc 14; and secondly additional room is accomplished within the ring 17by reason of linear shrinkage of the color glass disc 14 when it isannealcd subsequent to the assembly of the mounting ring 17.

The expansion of the disc 14 when the diametric crack is formed thereinis obvious, and it will be readily apparent as the ring 17 is spun overthe edge of the color glass disc 14 (or is bent over the edge of thedisc 14 by a forming die), that the glass cannot be compressed to occupyas small a space as prior to the forming of the diametric crack.

With respect to the heat treatment, the temperature to which the disc 14and its mounting ring 17 are heated is substantially above the normalannealed range of the glass, and the cooling of the disc 14 quickly instill air after it has been subjected to this high temperature iseflective to reduce the linear dimensions of the glass to an extent,according to tests made by the United States Bureau of Standards, whichmay amount to from 15 to 18 microns per centimeter of length. Because ofheat resistant glass of this character having a lower coeflicient ofexpansion than aluminum or copper, it will be apparent that the heatingduring the high temperature of the disc 14 when mounted within the ring1'7 does not set up stresses in the disc 14 because of the presence ofthe ring, as the heat is applied to the ring as well as to the glass.Upon cooling, however, subsequent to the heat treatment, the disc 14 isreduced in its linear dimension in the amount of from 15 to 18 micronsper centimeter, while the metal ring merely restores when cooled to itsoriginal dimensions. The disc 14 when thus treated undergoes a shrinkagewhich is permanent, and thus, upon subsequent heating in a signal, itnever restores to its original linear dimension. It is thereforeprovided that because of the shrinkage in the linear dimension of thedisc 14, space is obtained for expansion of the disc 14 within itsmounting ring 17 when it becomes heated by the light beam within asignal so that the disc 14 can expand without stresses being set up dueto the limiting efiect of the ring 17 which has been spun over one edgeof the glass. It will thus be seen that if a shrinkage is provided bythe heat treatment of from 15 to 18 microns per centimeter in lineardimension, the shrinkage substantially, if not entirely, compensates forthe expansion of the color glass disc 14 due to temperature change whenheated within a single so that the metal ring 17 in which the disc 14 ismounted, which is not heated by the light beam of the signal, does notlimit the expansion of the disc 14 so as to cause stresses to be set upopposing the expansion of the disc 14 due to temperature change.

Because the shrinkage of the disc 14 due to high temperature heattreatment may not always fully compensate for the expansion of the disc14 when used within a signal, it can be said that the additional spacefor expansion obtained within the ring 17 for the disc 14 by thediametric crack provides a safety margin insuring that there will be nostresses set up within the disc 14 when used within a signal due tocompression. by the ring 17 which fails to expand materially. Thereforethe color glass disc 14 may or may not be required to have a diametriccrack formed therein in accordance with the extent to which the disc 14is to be heated in a signal. That is, if the wattage of the lamp of asignal is to be only slightly increased over that which the color glassdisc 14, without a diametric crack and without high temperature heattreatment subsequent to assembly of the ring 17 will stand, it may bethat the color glass disc 14 need be only heat treated in order toprevent breakage. On the other hand, if the wattage of the signal lampis to be materially increased over that which the heat resistant glassdisc 14 will normally stand without a diametric crack and without beingannealed subsequent to the assembly of the ring 17, both the diametriccracking and the high temperature annealing process may be employed topermit maximum light intensity to be provided by the signal in which theglass is to be used without danger of breakage of the color glass discs14.

If a color glass disc 14 is to be annealed at the 1200 F. temperatureafter being assembled within a mounting ring 17, without having adiametric crack formed therein prior to mounting, the disc 14 may beidentified when viewed in the polariscope by the substantially squaredark pattern illustrated in Fig. 7, such pattern being indicative ofinternal stresses having been set up in the glass due to the hightemperature annealing and quick cooling. When this glass disc 14 withoutthe diametric crack is subsequently used in a signal, new stresses areset up so that the cooled glass disc 14 when viewed in a polariscopeafter having been used in a signal has a pattern as indicated in Fig. 8wherein four light areas are formed comparable to the respective leavesof a four leaf clover.

As has been indicated in the drawings, the character which the glassdiscs have assumed due to their heat treatment is distinctivelyindicated by stress patterns visible in a polariscope. The heatresistant glass from which the discs are cut has been annealed withinthe proper an nealing range to remove stresses and therefore it isisotropic in character as is indicated by there being no strain patternvisible when viewed in a polariscope. When thus annealed the glass is ofa particular character facilitating its being cut into discs 14, and itis also of such a character as to readily form the diametric crack 18 bybringing the respective discs 14 into contact with a hot wire.

After the glass has been annealed at 1200 F., however, and cooledrapidly in still air as has been specified, it is no longer isotropic incharacter, but instead there are double refractions causing patterns oflight and shaded areas to be visible when viewed in a polariscope. Ashas been pointed out, the pattern formed in the polariscope isindicative of the change in character of the discs 14 due to theparticular heat treatment to which the discs 14 have been subjected. ithas been found that it is consideraoly more diflicult to form thediametric crack 18 in the disc 14 after the high temperature annealinghas been done, and thus in addition to it being desirable that thediametric crack be formed prior to spinning of the ring 17 onto the disc14, the diametric crack 18 is more readily formed prior to the hightemperature annealing process.

In setting the high temperature annealing to be at 1200 F. for two andone-half minutes, there are other considerations aside from theshrinkage of the disc 14. Although higher temperatures could be used itit were only the glass to be considered, the temperature employedapproaches the melting point of the aluminum ring 17, and thus thetemperature cannot be materially increased above this point. Anotherconsideration is that a longer period of heating could well be employedexcept that a slight fading of the color of the discs 14 is noted as thetime of heat treatment is increased. Therefore, taking these mattersinto consideration, it has been found that the specified time andtemperature of annealin is preferable, although other times andtemperatures may be employed Without the character of the discs 14 asindicated by the stress patterns being materially modified.

it thus provided according to the present invention 'nproved resistanceto breakage of mounted colored disc 14 can be provided by heat treatmentat a erature above the normal annealing range of the ass and that aglass disc 14 so treated can be identified the particular shaded areasindicative of internal stresses which can be seen when the glass isviewed in a polariscope.

Having thus described a mounted glass disc and a method of heattreatment of such disc as one embodiment of crack in the disc, mountingthe disc in a circumisr metal channel, heating the mounted disc and itsassoc metal channel to approximately 1200 F. mately two and one-halfminutes, and cooling the L. ed disc in still air at room temperature.

2. The method of manufacture of a mounted glass disc to render such discheat resistant comprising forming a diametric crack in the disc,mounting the dis: in a circumferential metal channel, heating themounted disc and its associated metal channel to a temperature above thenormal annealing range of the glass for a relatively short length oftime, and cooling the mounted disc in still air at room temperature.

3. The method of manufacture of a mounted glass disc to render it heatresistant comprising mounting the disc in circumferential metal channel,heating the mounted disc and its associated metal channel toapproximately 1- for substantially two and one-half minutes, and coolingthe mounted disc in still air.

References Cited in the file of this patent UNITED STATES PATENTS

3. THE METHOD OF MANUFACTURE OF A MOUNTED GLASS DISC TO RENDER IT HEATRESISTANT COMPRISING MOUNTING THE DISC IN A CIRCUMFERENTIAL METALCHANNEL, HEATING THE MOUNTED DISC AND ITS ASSOCIATED METAL CHANNEL TOAPPROXIMATELY 1200* F. FOR SUBSTANTIALLY TWO AND ONE-HALF MINUTES, ANDCOOLING THE MOUNTED DISC IN STILL AIR.